1. Field of the Invention
This invention relates, in general, to power line carrier communication systems and, more specifically, to distribution network power line communication systems which transmit carrier current communication signals through commonly grounded neutral line conductors.
2. Description of the Prior Art
It has been found desirable to improve techniques for establishing communication links through the power conductors of a power distribution system. These communication links provide communication data and/or control information between the customers of an electrical utility company and a central communication terminal.
When the central communication terminal is located at a substation site, communication signals can be transmitted through the power line conductors of a distribution network to the remote terminals at individual customer premises. This avoids expensive and alternate communication transmission lines and/or radio link systems.
Some conventional systems include transmitter and receive circuits which impress and detect high-frequency carrier signals between pairs of power line conductors of a distribution network. Frequency translating and signal reconditioning repeaters are used to amplify and retransmit the power line communication signal through common pairs of power line conductors forming the communication links.
It has been recognized that the neutral line conductors of a distribution network provide a continuous unbroken metallic conduction path between a substation and subdistribution points, including distribution transformers, and to the secondary distribution portions serving individual utility company customers. These neutral line conductors are connected to real earth ground at multiple grounding points by grounding conductors located at the customer premises, the distribution points, and at various other equipment locations associated with the distribution network, as well as at the substations which feed the distribution networks.
A power line carrier communication system is described in U.S. Pat. No. 3,702,460, filed Nov. 30, 1971 by J. B. Bloose, in which communication signals are impressed onto a neutral conductor and a phase conductor. The phase conductor is also referred to as a line or hot conductor as distinguished from the neutral conductor which is connected to the system ground distribution network. The communication links for the communication signals are established between points where the neutral conductor is isolated from the true earth grounding points at the communication signal frequency being used. This isolation is provided by a separate circuit network. Thus, the conductors, including the neutral line which provides the transmission link for communication signals, are isolated from the true earth ground. This requires expensive isolating circuit arrangements to afford the communication links desired between remote terminals, such as provided at customer premises and at a substation site.
Therefore, it is desirable, and it is an object of this invention, to provide a communication system for power line networks which uses only the grounded uninterrupted neutral conductor of the power distribution network to provide the metallic conductor portion of the communication link.